The invention relates generally to electrodischarge machining (EDM) processes. More particularly, the invention relates to plunge EDM for making a honeycomb extrusion die.
Honeycomb bodies used in particulate filtration applications, such as diesel exhaust filtration applications, consist of a monolith body having longitudinal, parallel channels defined by longitudinal interconnected webs. The honeycomb bodies are typically made by extrusion from a ceramic material such as cordierite or silicon carbide. Extrusion dies used in making the honeycomb bodies have a die body with a discharge end including an array of longitudinal pins defined by interconnected slots. The array of longitudinal pins may include pins having any geometry useful in particulate filtration applications, such as square, triangle, or hexagon. The inlet end of the die body includes feedholes, which extend from the base of the die body to the slots and are used to supply batch material to the slots. To make a honeycomb body using the extrusion die, batch material is supplied to the feedholes and extruded through the slots. The batch material extruded through the slots form the interconnected webs of the honeycomb body.
Honeycomb extrusion dies are commonly made by plunge EDM. In a typical plunge EDM process, a shaped electrode having the desired pin/slot pattern is placed in contact with a workpiece. A voltage is applied across the shaped electrode and the workpiece to cause current to flow between them through a fluid electrolyte. In some processes, the shaped electrode acts as a cathode and the workpiece acts as an anode; in other processes, and depending under some circumstances on the composition of the electrode, the current flow may be reversed. The pin/slot pattern is formed in the workpiece by a series of repetitive electrical charges discharged in a thin gap between the shaped electrode and the workpiece. The electrical charges generate enough heat to melt the workpiece and transfer the pin/slot pattern to the workpiece. While machining the workpiece, the workpiece is immersed in dielectric fluid, which acts as a conductor for the electrical charges and at the same time insulates the shaped electrode from the workpiece. The dielectric fluid also serves as a coolant and is used to flush machined chips out of the thin gap between the shaped electrode and the workpiece.
While plunge EDM lends itself well to making machining pins and slots of various shapes and sizes within the same die body, a significant amount of variation in slot width and pin size within a machined die body has also been observed. This may be due to inadequate flushing of machined chips while forming the pin/slot pattern. Unflushed chips in a machining zone of the workpiece can create secondary discharges that could affect the width of the slots and size of the pins formed in that zone. Furthermore, the amount of unflushed chips varies from one machining zone to the next so that variation in slot width and pin size is not uniform across the workpiece, making it difficult to compensate for the effect of secondary discharges on slot width and pin size. Variation in slot width and pin size across the extrusion die translates to variation in web thickness and cell size across the honeycomb body formed with the extrusion die. Significant variation in web thickness and cell size across the honeycomb body can create an undesired thermal distribution profile and/or flow distribution profile in the honeycomb body during subsequent processing or use of the honeycomb body.
Thus there is a desire for a plunge EDM process that forms patterns in a workpiece with uniform variation in pattern size across the workpiece, wherein the variation can be controlled to meet manufacturing tolerances.